Microscale Technologies Research Articles

Can Integrated Micro-Optical Concentrator Technology Revolutionize Flat-Plate Photovoltaic Solar Energy Harvesting?

The economies-of-scale and enhanced performance of integrated micro-technologies have repeatedly delivered disruptive market impact. Examples range from microelectronics to displays to lighting. However, integrated micro-scale technologies have yet to be applied in a transformational way to solar photovoltaic panels. The recently announced Micro-scale Optimized Solar-cell Arrays with Integrated Concentration (MOSAIC) program aims to create a new paradigm in solar photovoltaic panel technology based on the incorporation of micro-concentrating photo-voltaic (μ-CPV) cells. As depicted in Figure 1, MOSAIC will integrate arrays of micro-optical concentrating elements and micro-scale PV elements to achieve the same aggregated collection area and high conversion efficiency of a conventional (i.e., macro-scale) CPV approach, but with the low profile and mass, and hopefully cost, of a conventional non-concentrated PV panel. The reduced size and weight, and enhanced wiring complexity, of the MOSAIC approach provide the opportunity to access the high-performance/low-cost region between the conventional CPV and flat-plate (1-sun) PV domains shown in Figure 2. Accessing this portion of the graph in Figure 2 will expand the geographic and market reach of flat-plate PV. This talk reviews the motivation and goals for the MOSAIC program. The diversity of the technical approaches to micro-concentration, embedded solar tracking, and hybrid direct/diffuse solar resource collection found in the MOSAIC portfolio of projects will also be highlighted.

Mechanistic Considerations for Enhancing Flow Boiling Heat Transfer in Microchannels

Research efforts on flow boiling in microchannels were focused on stabilizing the flow during the early part of the last decade. After achieving that goal through inlet restrictors and distributed nucleation sites, the focus has now shifted on improving its performance for high heat flux dissipation. The recent worldwide efforts described in this paper are aimed at increasing the critical heat flux (CHF) and reducing the pressure drop, with an implicit goal of dissipating 1 kW/cm2 for meeting the high-end target in electronics cooling application. The underlying mechanisms in these studies are identified and critically evaluated for their potential in meeting the high heat flux dissipation goals. Future need to simultaneously increase the CHF and the heat transfer coefficient (HTC) has been identified and hierarchical integration of nanoscale and microscale technologies is deemed necessary for developing integrated pathways toward meeting this objective.

Propagation Behaviors of an Acid Wavefront Through a Microchannel Junction.

Waves in reaction-diffusion systems yield a wealth of dynamic self-assembling phenomena in nature. Recent studies have been devoted to utilizing these active waves in conjunction with microscale technology. To provide a compass for controlling reaction-diffusion waves in microspaces, we have investigated the propagation behavior of one specific variety of the reaction-diffusion wave: an acid wave that utilizes an autocatalytic proton-production reaction. Furthermore, the acid wave that we have investigated occurs in a microchannel with a junction connecting circular and straight regions. The obtained results were compared with a neutralization wave that involves only a neutralization reaction. The acid wave was ignited by the addition of the appropriate amount of H2SO4 into the circular region that was filled with a substrate solution, where proton-consuming and proton-producing reactions followed a rapid neutralization reaction. At this stage, the wave penetrated and propagated into the channel region. Comparison between the acid and the neutralization waves clarified that the acid wave required a minimum threshold of H2SO4 concentration in order to be ignited and that the propagation of the acid wave was temporarily delayed because of the presence of intermediate chemical reaction steps. Furthermore, the propagation dynamics was found to be tuned through the configuration of the microchannel. The importance of microchannel configuration, especially for systems with a junction connecting different shapes, is discussed in terms of Fick's law and in terms of the proton flux from the circular to the straight regions.

Micro-scale aerosol-jet printing of graphene interconnects

This article addresses the deployment and characterization of a micro-scale aerosol-jet additive manufacturing technology to print graphene interconnects. A highly concentrated graphene ink with viscosity of 21cP and 3.1mg/ml graphene flakes with the lateral size below 200nm was developed and adopted for this process to make a reliable and repeatable graphene deposition on the treated Si/SiO2 wafers. To this end, the influence of the most significant process parameters, including the atomizer power, the atomizer flow rate, and the number of the printed layers, on the size and properties of graphene interconnects was studied. Results show that the aerosol-jet printing process is capable of printing micro-scale graphene interconnects with variable widths in the range of 10–90μm. These patterns, as the finest printed graphene patterns, with resistivity as low as 0.018Ωcm and sheet resistance of 1.64kΩ/□ may ease the development of miniaturized printed electronic applications of graphene.

Patients are a virtue: advances in microengineered systems for clinical applications.

Microscale technologies have been widely used in modern biology over the last decade, and have promised to revolutionize life sciences and biomedicine. However, the majority of microdevices developed to date have largely remained in the laboratory, with only a small fraction being applied in the field or clinic where their true impact on human health is best evaluated. Recently, microscale technologies have seen a rise in usage in applications with a clinical focus, indicating growing interest in this research direction. Here, we highlight the latest developments in microscale technologies designed specifically to handle, interrogate, and analyze human clinical samples for diagnostics and other applications.

Electrohydrodynamic direct-writing lithography: An alternative maskless technique for microstructure fabrication

A maskless electrohydrodynamic direct-writing lithographic strategy was presented to flexibly fabricate user-specific micropatterns on silicon substrates. By optimizing the operating parameters, parallel lines as well as lattices with line width of about 2 μm could be stably deposited. The printed micropatterns were found to function as sacrificial template to transfer microstructures into silicon substrates and the etching processes had little effect on the predefined size. It is envisioned that this simple approach provides an alternative to the existing microfabrication techniques, which might enable the wide accessibility of microscale technologies to advance various research fields such as microfluidics, biomedical chips, and microscale tissue engineering.

Microscale screening systems for 3D cellular microenvironments: platforms, advances, and challenges.

The increasing interest in studying cells using more in vivo-like three-dimensional (3D) microenvironments has created a need for advanced 3D screening platforms with enhanced functionalities and increased throughput. 3D screening platforms that better mimic in vivo microenvironments with enhanced throughput would provide more in-depth understanding of the complexity and heterogeneity of microenvironments. The platforms would also better predict the toxicity and efficacy of potential drugs in physiologically relevant conditions. Traditional 3D culture models (e.g., spinner flasks, gyratory rotation devices, non-adhesive surfaces, polymers) were developed to create 3D multicellular structures. However, these traditional systems require large volumes of reagents and cells, and are not compatible with high-throughput screening (HTS) systems. Microscale technology offers the miniaturization of 3D cultures and allows efficient screening of various conditions. This review will discuss the development, most influential works, and current advantages and challenges of microscale culture systems for screening cells in 3D microenvironments.

The relevance and potential roles of microphysiological systems in biology and medicine.

Microphysiological systems (MPS), consisting of interacting organs-on-chips or tissue-engineered, 3D organ constructs that use human cells, present an opportunity to bring new tools to biology, medicine, pharmacology, physiology, and toxicology. This issue of Experimental Biology and Medicine describes the ongoing development of MPS that can serve as in-vitro models for bone and cartilage, brain, gastrointestinal tract, lung, liver, microvasculature, reproductive tract, skeletal muscle, and skin. Related topics addressed here are the interconnection of organs-on-chips to support physiologically based pharmacokinetics and drug discovery and screening, and the microscale technologies that regulate stem cell differentiation. The initial motivation for creating MPS was to increase the speed, efficiency, and safety of pharmaceutical development and testing, paying particular regard to the fact that neither monolayer monocultures of immortal or primary cell lines nor animal studies can adequately recapitulate the dynamics of drug-organ, drug-drug, and drug-organ-organ interactions in humans. Other applications include studies of the effect of environmental toxins on humans, identification, characterization, and neutralization of chemical and biological weapons, controlled studies of the microbiome and infectious disease that cannot be conducted in humans, controlled differentiation of induced pluripotent stem cells into specific adult cellular phenotypes, and studies of the dynamics of metabolism and signaling within and between human organs. The technical challenges are being addressed by many investigators, and in the process, it seems highly likely that significant progress will be made toward providing more physiologically realistic alternatives to monolayer monocultures or whole animal studies. The effectiveness of this effort will be determined in part by how easy the constructs are to use, how well they function, how accurately they recapitulate and report human pharmacology and toxicology, whether they can be generated in large numbers to enable parallel studies, and if their use can be standardized consistent with the practices of regulatory science.

Microscale Generation of Cardiospheres Promotes Robust Enrichment of Cardiomyocytes Derived from Human Pluripotent Stem Cells

SummaryCardiomyocytes derived from human pluripotent stem cells (hPSCs) are a promising cell source for regenerative medicine, disease modeling, and drug discovery, all of which require enriched cardiomyocytes, ideally ones with mature phenotypes. However, current methods are typically performed in 2D environments that produce immature cardiomyocytes within heterogeneous populations. Here, we generated 3D aggregates of cardiomyocytes (cardiospheres) from 2D differentiation cultures of hPSCs using microscale technology and rotary orbital suspension culture. Nearly 100% of the cardiospheres showed spontaneous contractility and synchronous intracellular calcium transients. Strikingly, from starting heterogeneous populations containing ∼10%–40% cardiomyocytes, the cell population within the generated cardiospheres featured ∼80%–100% cardiomyocytes, corresponding to an enrichment factor of up to 7-fold. Furthermore, cardiomyocytes from cardiospheres exhibited enhanced structural maturation in comparison with those from a parallel 2D culture. Thus, generation of cardiospheres represents a simple and robust method for enrichment of cardiomyocytes in microtissues that have the potential use in regenerative medicine as well as other applications.

Microscale technologies for regulating human stem cell differentiation.

During development and regeneration, tissues emerge from coordinated sequences of stem cell renewal, specialization, and assembly that are orchestrated by cascades of regulatory factors. This complex in vivo milieu, while necessary to fully recapitulate biology and to properly engineer progenitor cells, is difficult to replicate in vitro. We are just starting to fully realize the importance of the entire context of cell microenvironment-the other cells, three-dimensional matrix, molecular and physical signals. Bioengineered environments that combine tissue-specific transport and signaling are critical to study cellular responses at biologically relevant scales and in settings predictive of human condition. We therefore developed microbioreactors that couple the application of fast dynamic changes in environmental signals with versatile, high-throughput operation and imaging capability. Our base device is a microfluidic platform with an array of microwells containing cells or tissue constructs that are exposed to stable concentration gradients. Mathematical modeling of flow and mass transport can predict the shape of these gradients and the kinetic changes in local concentrations. A single platform, the size of a microscope slide, contains up to 120 biological samples. As an example of application, we describe studies of cell fate specification and mesodermal lineage commitment in human embryonic stem cells and induced pluripotent stem cells. The embryoid bodies formed from these cells were subjected to single and multiple concentration gradients of Wnt3a, Activin A, bone morphogenic protein 4 (BMP4), and their inhibitors, and the gene expression profiles were correlated to the concentration gradients of morphogens to identify the exact conditions for mesodermal differentiation.

Radiative properties tailoring of grating by comb-drive microactuator

Micro-scale grating structures are widely researched in recent years. Although micro-scale fabrication technology is highly advanced today, with grating aspect ratio greater than 25:1 being achievable some fabrication requirements, such as fine groove processing, are still challenging. Comb-drive microactuator is proposed in this paper to be utilized on simple binary grating structures for tailoring or modulating spectral radiation properties by active adjustment. The rigorous coupled-wave analysis (RCWA) is used to calculate the absorptance of proposed structures and to investigate the impacts brought by the geometry and displacement of comb-drive microactuator. The results show that the utilization of comb-drive microactuator on grating improves the absorptance of simple binary grating while avoiding the difficulty fine groove processing. Spectral radiation property tailoring after gratings are fabricated becomes possible with the comb-drive microactuator structure.

Effect of Finite Diaphragm Rupture Process on Microshock Tube Flows

The study of flow physics in microshock tubes is of growing importance with the recent development of microscale technology. The flow characteristics in a microshock tube is considerably different from that of the conventional macroshock tube due to the boundary layer effects and high Knudsen number effects. In the present study an axisymmetric computational fluid dynamics (CFD) method was employed to simulate the microshock tube flow field with Maxwell's slip velocity and temperature jump boundary conditions, to accommodate the rarefaction effects. The effects of finite diaphragm rupture process and partial diaphragm rupture on the flow field and the wave propagations were investigated, in detail. The results show that the shock propagation distance attenuates rapidly for a microshock tube compared to a macroshock tube. For microshock tubes, the contact surface comes closer to the shock front compared to the analytical macroshock tube case. Due to the finite diaphragm rupture process the moving shock front will be generated after a certain distance ahead of the diaphragm and get attenuated rapidly as it propagates compared to the sudden rupture case. The shock-contact distance reduces considerably for the finite diaphragm rupture case compared to the sudden diaphragm rupture process. A partially burst diaphragm within a microshock tube initiates a supersonic flow in the vicinity of the diaphragm similar to that of a supersonic nozzle flow. The supersonic flow expansion leads to the formation of oblique shock cells ahead of the diaphragm and significantly attenuates the moving shock propagation speed.

Fabrication of a 3D hair follicle‐like hydrogel by soft lithography

Hair follicle transplantation is often used in the treatment of androgenetic alopecia (AGA). However, the only source of hair follicles is from human donors themselves, which limits the application of this approach. One possible solution is to reconstitute hair follicle from dissociated cells. Currently, a number of microscale technologies have been developed to create size and shape controlled microenvironments in tissue engineering. Photopolymerizable PEGDA hydrogels are often selected as promising scaffolds in engineered microtissues due to their biocompatibility and adjustable mechanical properties. Here, we fabricated an array of PEGDA microwells with center islets that mimic the architecture of human hair follicles using soft lithography. Dermal and epithelial cells were seeded in different compartments of the microstructured mould to mimic mesenchymal and epithelial compartmentalization in native hair follicles. We demonstrated that these compartmentalized microstructures support cell proliferation and cell survival over 14 days, and spreading of dermal fibroblasts was observed. This hydrogel micromould provides a potentially useful tool for engineering 3D hair follicle-mimicking complex cultures in vitro.

A feasibility integrated approach in the promotion of solar house design

SUMMARY In many countries, although solar energy is plentiful, due to technical and economic issues, micro-scale solar energy technologies have not properly found their places in the market. The present work demonstrates the convenience of employing economic feasibility assessments together with engineering analyses before applying solar strategies during the design stages, in order to convince house owners that these solar technologies can be economically viable. In a case study, this approach is tested for a real house project developed in North Cyprus (NC) in which solar energy applications for electricity generation, hot water preparation, space heating and passive cooling are investigated. It is reported that grid-connected photovoltaic (PV) systems would be economically feasible with savings-to-investment ratio (SIR) values of 2.9 to 3.4. A thermal performance analysis is carried out for a thermal storage wall (TSW) together with a direct solar gain (DSG) window in the living room. It is discovered that the temperature range of the living room was within 18–22 °C throughout the majority of winter days. This application is also proved to be economically feasible with an SIR of 1.3, compared to installing an 18,000 Btu/h heat pump. It is determined that both locally produced solar water heaters and imported solar water heaters (ISWHs) are economically viable compared to using a 3-kW storage-type electric water heater. Their SIR values are estimated to be 7.5 and 3.7, respectively. A solar-assisted stack-effect ventilation (SEV) system is verified to have techno-economic feasibility and is also useful in concealing the unattractive cold and hot water storage tanks on the roof. The prospective house owner decides to invest 3840 EUR in installing a grid-connected PV system of 1 kWp, an ISWH, a DSG window, building a TSW for space heating and a SEV system with an overall SIR value of 3.2. This procedure can be developed into a government policy where all these assessments are made mandatory to increase the awareness of the home builders before they make their decisions about the final designs of their houses. The present work demonstrates the convenience of employing economic feasibility assessments together with engineering analyses before applying solar strategies during the design stages, in order to convince house owners that these solar technologies can be economically viable. In a case study, this approach is tested for a real house project developed in NC in which solar energy applications for electricity generation, hot water preparation, space heating and passive cooling are investigated. Copyright © 2013 John Wiley & Sons, Ltd.

Microfluidic bioreactor for dynamic regulation of early mesodermal commitment in human pluripotent stem cells

During development and regeneration, tissues emerge from coordinated sequences of stem cell renewal, specialization and assembly that are orchestrated by cascades of regulatory signals. The complex and dynamic in vivo milieu cannot be replicated using standard in vitro techniques. Microscale technologies now offer potential for conducting highly controllable and sophisticated experiments at biologically relevant scales, with real-time insights into cellular responses. We developed a microbioreactor providing time sequences of space-resolved gradients of multiple molecular factors in three-dimensional (3D) cell culture settings, along with a versatile, high-throughput operation and imaging compatibility. A single microbioreactor yields up to 120 data points, corresponding to 15 replicates of a gradient with 8 concentration levels. Embryoid bodies (EBs) obtained from human embryonic and induced pluripotent stem cells (hESC, hiPSC) were exposed to concentration gradients of Wnt3a, Activin A, BMP4 and their inhibitors, to get new insights into the early-stage fate specification and mesodermal lineage commitment. We were able to evaluate the initiation of mesodermal induction by measuring and correlating the gene expression profiles to the concentration gradients of mesoderm-inducing morphogens. We propose that the microbioreactor systems combining spatial and temporal gradients of molecular and physical factors to hESC and hiPSC cultures can form a basis for predictable in vitro models of development and disease.

Separation technologies for stem cell bioprocessing

Stem cells have been the focus of an intense research due to their potential in Regenerative Medicine, drug discovery, toxicology studies, as well as for fundamental studies on developmental biology and human disease mechanisms. To fully accomplish this potential, the successful application of separation processes for the isolation and purification of stem cells and stem cell-derived cells is a crucial issue. Although separation methods have been used over the past decades for the isolation and enrichment of hematopoietic stem/progenitor cells for transplantation in hemato-oncological settings, recent achievements in the stem cell field have created new challenges including the need for novel scalable separation processes with a higher resolution and more cost-effective. Important examples are the need for high-resolution methods for the separation of heterogeneous populations of multipotent adult stem cells to study their differential biological features and clinical utility, as well as for the depletion of tumorigenic cells after pluripotent stem cell differentiation. Focusing on these challenges, this review presents a critical assessment of separation processes that have been used in the stem cell field, as well as their current and potential applications. The techniques are grouped according to the fundamental principles that govern cell separation, which are defined by the main physical, biophysical, and affinity properties of cells. A special emphasis is given to novel and promising approaches such as affinity-based methods that take advantage of the use of new ligands (e.g., aptamers, lectins), as well as to novel biophysical-based methods requiring no cell labeling and integrated with microscale technologies.

Material considerations forin vitroneural interface technology

Abstract

Analysis of Caenorhabditis elegans in microfluidic devices

Caenorhabditis elegans (C. elegans) is widely adopted as a model organism for a variety of biological studies including development, genetics and neurobiology. Micro-scale microfluidic technology is capable of handling single or populations of C. elegans in high throughput format and allows for the precise spatial and temporal control of their environment, which is well suited for the study of worms in different aspects. In this review, we highlight the recent advances in microfluidic technology for the analysis of worms ranging from behavioral studies to neurobiology. We believe that microfluidic device can further be applied to study the different aspects of worms, extending from fundamental investigation of behavioral dynamics to more complicated biological processes including neurochemistry and learning behaviors.

Gold nanoparticle-based optical microfluidic sensors for analysis of environmental pollutants

Conventional methods of environmental analysis can be significantly improved by the development of portable microscale technologies for direct in-field sensing at remote locations. This report demonstrates the vast potential of gold nanoparticle-based microfluidic sensors for the rapid, in-field, detection of two important classes of environmental contaminants - heavy metals and pesticides. Using gold nanoparticle-based microfluidic sensors linked to a simple digital camera as the detector, detection limits as low as 0.6 μg L(-1) and 16 μg L(-1) could be obtained for the heavy metal mercury and the dithiocarbamate pesticide ziram, respectively. These results demonstrate that the attractive optical properties of gold nanoparticle probes combine synergistically with the inherent qualities of microfluidic platforms to offer simple, portable and sensitive sensors for environmental contaminants.

Magnetic manipulation and spatial patterning of multi-cellular stem cell aggregates

The controlled assembly and organization of multi-cellular systems to mimic complex tissue structures is critical to the engineering of tissues for therapeutic and diagnostic applications. Recent advances in micro-scale technologies to control multi-cellular aggregate formation typically require chemical modification of the interface between cells and materials and lack multi-scale flexibility. Here we demonstrate that simple physical entrapment of magnetic microparticles within the extracellular space of stem cells spheroids during initial formation enables scaffold-free immobilization, translocation and directed assembly of multi-cellular aggregates across multiple length and time scales, even under dynamic suspension culture conditions. The response of aggregates to externally applied magnetic fields was a direct function of microparticle incorporation, allowing for rapid and transient control of the extracellular environment as well as separation of heterogeneous populations. In addition, spatial patterning of heterogeneous spheroid populations as well as individual multi-cellular aggregates was readily achieved by imposing temporary magnetic fields. Overall, this approach provides novel routes to examine stem cell differentiation and tissue morphogenesis with applications that encompass the creation of new model systems for developmental biology, scaffold-free tissue engineering strategies and scalable bioprocessing technologies.